object-store-ll.h: split this header out of object-store.h

The vast majority of files including object-store.h did not need dir.h
nor khash.h.  Split the header into two files, and let most just depend
upon object-store-ll.h, while letting the two callers that need it
depend on the full object-store.h.

After this patch:
    $ git grep -h include..object-store | sort | uniq -c
          2 #include "object-store.h"
        129 #include "object-store-ll.h"

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/commit-graph.h b/commit-graph.h
index 83aaa1d..5e534f0 100644
--- a/commit-graph.h
+++ b/commit-graph.h
@@ -1,7 +1,7 @@
 #ifndef COMMIT_GRAPH_H
 #define COMMIT_GRAPH_H
 
-#include "object-store.h"
+#include "object-store-ll.h"
 #include "oidset.h"
 
 #define GIT_TEST_COMMIT_GRAPH "GIT_TEST_COMMIT_GRAPH"